The present invention relates to an improved system for measuring selected performance characteristics of electronic components. In one preferred embodiment, the present invention comprises a method and apparatus for evaluating selected performance criteria of microwave power components, and in particular, microwave transmitter and receiver components. The present invention comprises improvements that individually or in combination increase the accuracy, reproducibility, and speed of these determinations, in a system that is lightweight, portable, and low cost, relative to prior known devices.
Microwave components have long been critical features of radar systems, electronic devices, and other systems. Errors in the parameters of microwave components translate directly into decreased accuracy and precision of the equipment, systems, and processes in which they are employed. There has long been a need to improve the accuracy, reliability, repeatability, and correlation of signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d) measurements of microwave power transmitter and receiver components. Prior to the present invention, a relatively high degree of variability existed between test sites, as well as between test sets at the same test site. Improvement in the accuracy of the performance characteristics of microwave components contributes directly to improved accuracy and precision in the systems in which they are used. Prior approaches have not adequately met this need. In response to this need, a self-calibrating measurement technique having improved speed, accuracy, repeatability, and portability of the present invention has been developed.
Many microwave transmitters have stringent requirements on their output signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d). For example, in a pulsed radar system, the transmitter SNR can limit the ability of the radar to detect and/or track weak targets against a background of echoes from the earth""s surface (clutter). The techniques commonly used prior to the present invention to measure transmitter and microwave tube SNR can have errors of xc2x11.5 dB. As a result, there has been poor agreement among SNR measurements made at the component factory, transmitter manufacturer, system integrator, and end user. This lack of agreement causes disputes over product acceptance, results in rejection of components meeting specification and acceptance of components failing to meet specification, creates unnecessary product returns, and generates wasteful requests to retest components. Moreover, the prior known methods for determining the performance characteristics of microwave power components, and in particular SNR, were: cumbersome; time consuming; expensive; and involved the use of large, heavy, bulky, and expensive equipment to carry out the analysis.
Prior to the present invention, microwave components have been tested in a test setup of the type depicted in the right-hand portion of FIG. 1. The right-hand portion of FIG. 1 depicts a test set up used for evaluating a component under test, in this example, a microwave power tube (or transmitter). A radio frequency (rf) drive signal was supplied to the component. The rf output from the component under test was applied to a vector demodulator circuit which, converted it into in-phase (I) and quadrature (Q) video signals. The rf drive signal to the component was also used as the reference (local oscillator) input to the demodulator as shown in FIG. 1. The signals were then digitalized and filtered. In the test setup depicted in FIG. 1, a Tektronix RTD-710A digitizer, sampling at 50 Msa/sec, was used to convert the I and Q video signals into 10-bit digital format. Analog 5 MHZ 6-pole Bessel filters at the inputs of the digitizer channels were used to define the bandwidth in which the tube noise is measured, i.e. xc2x15 MHZ about the carrier. For pulsed radar tubes, the digitized data was captured in a 2-microsecond window located near the middle of the pulse. The raw data was then transferred to a general purpose digital computer. Application software compensated the data for pulse-to-pulse variations caused by the modulator, and computed the average intrapulse SNR referenced to a 1 MHZ bandwidth. This conventional technique, however, took substantial amounts of time per measurement and resulted in SNR measurements with an error of xc2x11.5 dB.
The inventor observed that accuracy of prior known methods was limited by vector demodulator errors, which could be removed by lengthy calibration procedures. Specifically, there are both linear and non-linear error sources in the measurement of microwave power components.
Linear Distortion: An ideal vector demodulator (VDM) generates a unit circle centered at the origin on the IQ plane. The I and Q video output voltages are defined by the equations:
xe2x80x83I=kA cos xcex8
Q=kA sin xcex8
where
A=rf signal voltage
k=mixer conversion loss
xcex8=phase angle of rf signal with respect to the LO
Plotting I and Q data from a real vector demodulator generated an elliptical locus displaced from the origin as shown in FIG. 2. The inventor observed that this linear distortion resulted from three error sources:
DC Offsetxe2x80x94Imbalances in the mixer diodes and transformers create low level dc outputs in the I and Q channels when the local oscillator signal is applied. This causes displacement of the center of the locus from the origin. These offsets are a function of the measurement frequency and, if not compensated, cause a few tenths of a dB error.
I/Q Channel Gain Imbalancexe2x80x94This difference in gain between I and Q channels changes the locus into an ellipse with its principal axes parallel to the I and Q axes. Factors which contribute to gain imbalance include variations in:
rf power split into the I and Q channel mixers;
VSWR of the mixer rf ports;
mixer conversion loss;
insertion loss of video filters;
digitizer channel gains;
The first three items were considered by the present inventor to be dominant, and varied as a function of the test frequency.
Quadrature Errorxe2x80x94Quadrature error causes the principal axes of the elliptical locus to tilt relative to the I/Q axes and results from the I and Q channel phases differing from 90 degrees. Differential phase errors are caused by:
The rf power splitters used to supply RF and LO to the I and Q mixers;
Rf line lengths; and
Mixer VSWRs.
These errors are also functions of the test frequency. These three linear distortions were determined by the inventor to be the major causes of SNR measurement inaccuracy.
The distorted I and Q voltages take the form:       I    =                  kA        ⁢                  (                      1            -                          C              2                                )                ⁢                  xe2x80x83                ⁢                  cos          ⁡                      (                          θ              -                              D                2                                      )                              +              B        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢        E                  Q    =                  kA        ⁢                  (                      1            +                          C              2                                )                ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                          θ              +                              D                2                                      )                              +              B        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢        E            
where             xe2x80x83        ⁢          C      =              gain imbalance;                  D    =          quadrature error            B    =          dc offset amplitude            E    =          dc offset phase relative to the I axis                          C        dB            =              2        ⁢                  log          ⁡                      (                                          1                +                                  (                                      C                    2                                    )                                                            1                -                                  (                                      C                    2                                    )                                                      )                                ⁢          xe2x80x83      
FIG. 3 illustrates the linear distortion in a typical vector demodulator. A gain imbalance of xc2x11 dB and a phase error of xc2x16 degrees are typical values. These errors are attributed to mixer Voltage Standing Wave Ratios (xe2x80x9cVSWRsxe2x80x9d), which can be in the range 3.0:1. In addition, this vector demodulator (VDM) shows a bias in the phase of about xe2x88x928 degrees. This is attributed to a small differential error (c. 0.05 inches) in the rf line lengths feeding the I/Q mixers. The dc offset for this VDM is on the order of 1%, and is a small error source.
Non-Linear Distortionxe2x80x94Prior to the present invention, the LO to signal ratio typically was not tightly controlled. The inventor observed that non-linear distortion generated in the demodulator could be made negligible by maintaining the signal amplitude 15 dB below the LO reference level. The LO/signal ratio was typically only 8 dB, where considerable amplitude compression was readily observable.
The calibration procedure of prior known methods typically required considerable time (3-5 seconds or longer) and entailed relatively large errors (xc2x12dB). The procedure of the present invention speeds the assessment and removes these errors by implementing a local oscillator (LO) offset frequency technique, and digital signal processing of the video data. Specifically, the invention employs a Rotating Phase Reference (xe2x80x9cRPRxe2x80x9d) technique to improve accuracy. The invention may also use digital filters to create a precise noise bandwidth, and to significantly reduce the noise floor of the measurement. The technique of the present invention is suitable for use in both Government and commercial applications. The improved accuracy of the present invention results in far better agreement between tube vendors, equipment integrators, and the final customer, leading to lower costs by increasing equipment acceptance.
Others, prior to the present invention, have sought to develop methods of real-time measurement of signal-to-noise ratio. None, however, employ the RPR technique of the present invention.
One method for determining signal-to-noise ratio is described in Glomb, U.S. Pat. No. 3,101,446 for Signal-to-Noise Ratio Indicator (Aug. 20, 1963). Glomb discloses a signal-to-noise ratio indicator for a communication system that employs a combination of a low frequency sine wave and out of band noise.
Another patent issued to Taylor, U.S. Pat. No. 3,287,646 for Signal-To-Noise Ratio Meter (Oct. 28, 1963), discloses a meter for the direct measurement of the signal-to-noise ratio of a modulated continuous wave that has noise level superimposed on it. Taylor""s method involves mixing the incoming (noisy) signal with a different frequency oscillation and performing a series of limiting and filtering steps.
Newman, U.S. Pat. No. 3,743,932 for Clipped Correlation to Signal-to-Noise Ratio Meter (Jul. 3, 1973), discloses a circuit for converting the output of a clipper correlator to signal-to-noise ratio in a signal from a sonar array.
Hammett, U.S. Pat. No. 3,825,835 for Signal-to Noise Ratio Measurement (Jul. 23, 1974), discloses a system and apparatus for the measurement of signal-to-noise ratio in electrical signals. Hammett notes that the two prior methods known at that time for measurement of signal-to-noise ratio are complex and/or extremely bulky. Col. 1, at 11. 43-50. Although Hammett discloses what he describes as a xe2x80x9cportablexe2x80x9d system, that system employs cathode-ray oscilloscope equipment that is considerably more complex, bulky, costly, and heavy than the apparatus of the present invention.
Campbell, U.S. Pat. No. 4,004,230 for Critical Parameter Receiver Tester (Jan. 18, 1977), discloses test equipment on which a modulated signal is injected into a receiver under test. The output is fed directly to a first power measuring device, then through a digital filter, to a second measuring device. The two measured quantities are then fed to a divider circuit which provides an indication of the signal-to-noise ratio.
Tenten, U.S. Pat. No. 4,172,263 for Methods and Apparatus for Measuring Signal-to Noise Ratio (Oct. 23, 1979), discloses a method and apparatus for measuring signal-to-noise ratio in video waveforms, based upon tangential noise measurement principles.
Wong, U.S. Pat. No. 5,386,495 for Method and Apparatus for Determining the Signal Quality of a Digital Signal (Jan. 31, 1995), discloses a receiver for receiving and decoding a carrier signal. The carrier signal is modulated with a digital signal. The receiver signal is demodulated and converted into digital format. A digital signal processor is then used to calculate the signal-to-noise ratio.
Mueller, U.S. Pat. No. 5,465,412 for Apparatus and Method for Determining a Point in Time for Detecting a Sampled Signal in a Receiver (Nov. 7, 1995), discloses a system adapted for mobile communications systems.
None of the prior known methods and devices achieves the rapid, high resolution measurement of signal-to-noise ratio of the present invention. Moreover, none of them has been able to improve substantially the accuracy of prior measurement methods through a light weight, portable measurement device or system. The present invention overcomes many of the drawbacks of prior methods and greatly improves the accuracy, ease, and precision of measurement, its reproducibility, and results in greater reliability of the systems in which the components are used. Specifically, the technique of the present invention results in an accuracy of better than about 0.1 dB, in contrast to errors of xc2x11.5 dB, typical in prior techniques. The repeatability of the measurement of signal-to-noise ratio depends primarily on the number of independent noise samples taken. The present invention facilitates the rapid measurement of parameters and, hence, the measurement of a larger number of samples, improving accuracy. In addition, the apparatus of the present invention is faster, smaller, less expensive, and more robust and light weight than prior apparatus and methods.
The present invention is the result of several years"" development work. In a 1994 paper, entitled High Accuracy Signal/Noise Measurement of Microwave Power Amplifiers (the xe2x80x9c1994 paperxe2x80x9d), by the inventor, Alexander MacMullen, of Technology Service Corporation, and Steve Hillenberg, NSWC, Crane Division, the inventor""s contact at the U.S. Navy""s Crane Laboratory, the authors disclose many of the problems involved in improving the accuracy of measurements of the properties of microwave power components and outline a general conceptual path toward a solution. Papers describing the work were presented at the 1994 Microwave Power Tube Conference, the IEEE/MTTS Automatic RF Techniques Group (xe2x80x9cARFTGxe2x80x9d), and were published in NASA Technical Briefs, and elsewhere. That 1994 paper, as well as each of the IEEE/MTTS, ARFTG, and NASA papers, are hereby incorporated by reference, as if fully set forth herein. Although the 1994 paper describes, in general terms, the basic RPR concept involved in the system of the present invention to achieve an accuracy of better than 0.1 dB, it does not disclose the system, apparatus, or process of the present invention.
The 1994 paper discloses that an analysis of the demodulator circuit was performed to see how the major error sources could be eliminated or reduced. It was determined that the vector demodulator components were already of the highest quality commercially available and, therefore, it was impractical to improve their characteristics without tedious and time-consuming tuning, matching, and adjustment. Very high-level mixers were selected for the demodulator. These units, Anaren model 76127, were operated with an LO level of +18 dBm. The third order intercept point was above +24 dBm. With a signal level of +3 dBm, non-linear distortion products were reported to have been negligible.
The measurement procedure employed in the 1994 paper used a hardware configuration of the type shown in FIG. 1. In contrast to the prior technique, in which the rf drive signal was supplied to both the test component and as the local oscillator input to the vector demodulator, the LO reference signal was offset in frequency from the rf signal by a small amount, and data was collected over a time base T=N/xcex94f, where xcex94f=LO frequency offset and N is an integer. The data values were uniformly distributed over all values of xcex8 and any cyclic errors could be removed by averaging.
As an example, a pulsed S-band crossed-field amplifier tube was operated at a prf of 2000 Hz. The frequency offset was adjusted to 20 Hz. Data was collected over a time base of 0.05 seconds, or 100 pulses. Each pulse had an incremental phase of 3.60 degrees. Over the total data base, the accumulated phase was 360 degrees.
The digitized I and Q data were transferred to the computer where the mean values in each pulse window were calculated. The data were normalized to remove pulse-to-pulse variations which may have been generated by the pulse modulator. The variances of the normalized data were computed and summed to yield the total noise power in each pulse. Results from all pulses were averaged. Finally, the signal-to-noise ratio was computed for the entire group of pulses in the data base. The errors caused by linear distortion were removed because the average of sinusoidal functions over N cycles is zero.
Based upon this technique, the 1994 paper reports improved accuracy of SNR measurements on microwave tubes and transmitters of about 0.1 dB rms, compared to xc2x11.5 dB in previous known procedures. Nonetheless, the disclosure of the 1994 Paper retained a number of the drawbacks of prior techniques. Although the technique disclosed in the 1994 Paper was more accurate than prior known techniques, the process remained slow, on the average of 3 to 5 seconds per measurement. The instrumentation used was large, heavy, bulky, and expensive, industrial grade equipment. The system was not portable. Two microwave signal generators, operating at slightly offset frequencies, produced the Rotating Phase Reference (RPR). This had two distinct disadvantages: (1) the second microwave synthesizer was costly, and (2) the phase noise of the two synthesizers was cumulative, creating a high measurement noise floor. The only performance parameter evaluated in the 1994 paper was SNR. The number of measured phases was substantial. None of the details of the process, algorithms, or software employed were disclosed.
The inventor has made significant improvements in various elements and components of the work described in the 1994 Paper. The present invention represents a significant advance over prior know methods of signal-to-noise ratio measurement. High accuracy has been retained. Many of the functions have been converted from hardware- to software-based systems. The number of phases at which data are collected has been reduced. The present invention uses standard, off the shelf, readily available components, is more lightweight, and is easily portable. In addition, the present invention offers the significant improvements in the speed of measurement of signal-to-noise ratio. This improvement was made possible by two innovations: (1) the rapid transfer of data between components of the testing system; and (2) employing a special purpose, high speed, digital processor rather than a general purpose digital computer. The inventor has made additional improvements to speed up the processing of the raw data acquired during the RPR measurement and provide operators with an SNR vs. frequency display in two seconds. This represents an increase in speed of about 3000:1 relative to prior known techniques and several times the speeds reported in the 1994 paper.
The present invention is useful to accurately and rapidly measure and display SNR (and its related parameters, moving target indication (MTI) and clutter attenuation (CA), and other fundamental parameters of microwave components such as insertion phase, gain, and time delay.
It is therefore an object of the present invention to provide a process and apparatus to improve the accuracy of SNR measurements on microwave components, including but not limited to tubes, transmitters, and other electronic components.
Another object of the present invention is to improve the accuracy of SNR measurements to approximately xc2x10.1 dB, or better.
An object of the invention is to reduce the time required to make accurate signal-to-noise ratio measurements in less than 3 to 5 seconds.
A further object of the present invention is reduce errors in SNR measurement by employing a reference LO shifted in phase from the signal to the demodulator.
Yet, a further object of the present invention is the use of a Rotating Phase Reference (RPR) technique to improve the accuracy of determination of performance parameters of electronic components.
An additional object of the present invention is to improve the reproducibility of measurement of selected performance characteristics of microwave power components.
Yet another object of the present invention is to decrease the time required to measure selected performance characteristics, while maintaining a high degree of accuracy.
Another object of the present invention is to provide a system for the measurement of selected performance characteristics of microwave power components that is portable and light weight.
A further object of the present invention is to employ a Rotating Phase Reference (RPR) technique to improve the accuracy and speed of the measurement of selected performance characteristics of microwave power components.
An additional object of the present invention is to improve the accuracy and repeatability of measurement by automating data collection.
Yet another object of the present invention is to improve the accuracy of determination of performance parameters of electronic components by automating the processing of the collected data.
A further object of the present invention to provide a standard measurement technique and apparatus for the determination of selected performance characteristics of microwave power components.
Another object of the present invention is to provide a rugged measurement system that is portable.
Yet another object of the invention is to provide a measurement system that is light weight.
A further object of the present invention is to improve the accuracy of phase, gain, signal to noise, time delay measurements, and system stability.
Another object of the present invention is to provide a system for determining performance parameters of electronic components that weighs less than about 120 pounds.
An additional object of the present invention is to provide a system for determining performance parameters of electronic components that weights less than about 50 pounds.
A further object of the present invention is to provide a system for determining performance parameters of electronic components having a package that is smaller than about 3,000 cubic inches.
An additional object of the present invention is to provide a system that employs improved accuracy based upon an increased number of measurements at a reduced number of phases.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention will be realized in detail by means of the instrumentalities and combinations particularly pointed out in the appended claims.
As illustrated in the accompanying diagrams and disclosed in the accompanying claims, the invention is a method for the measurement of selected fundamental performance parameters of test unit, comprising:
a. generating a first reference signal;
b. generating a second reference signal, that is rotated in phase relative to said first reference signal;
c. Supplying said first reference signal to the test unit;
d. Supplying the output of the test unit to vector demodulator means;
e. Supplying said second reference signal to vector demodulator means;
f. Generating a signal from vector modulator means;
g. Processing said signal from said vector demodulator means to determine the selected fundamental performance parameters of the test unit.
In another embodiment, the present invention is an apparatus for the measurement of fundamental parameters of a test unit, comprising:
a. first signal generator means, for generating a first signal having first phase;
b. phase shifter means, cooperating with said first signal generator means, for rotating the phase of said first signal to produce a second signal having a second phase, wherein said second signal is shifted in phase relative to said first signal;
c. test unit input means, for supplying said first signal to said test unit input means,
d. demodulator means, for accepting an rf signal from the test unit,
e. means for supplying said second signal to said demodulator means,
f. data processing means for computing the fundamental parameter of the test unit from the output of said digitizer.
The invention is a PC-based test instrument that performs high accuracy, quantitative measurements of microwave components over the 2 to 18 Ghz frequency band. Used with a conventional synthesizer that sweeps over user-specified frequencies, the MCA analyzes the CW or pulse output from the device under test and generate the following measures: Signal-to-Noise Ratio; Insertion Gain; and Insertion Phase. The invention includes the MCA, a GPIB compatible synthesizer, a power divider, and the device under test (DUT). The MCA controls the synthesizer sweep using gain and frequency parameters defined by the operator. The power divider provides the RF stimulus to the DUT and a reference signal for the MCA.
The invention measures the output Signal-to-Noise Ratio (SNR), insertion gain and phase of the DUT under continuous wave (CW) or pulsed conditions. For pulsed waveforms, the invention synchronizes its measurements to an externally supplied trigger. For CW waveforms, the invention generates its own internal 1 KHz trigger. The MCA operates at discrete frequencies over any portion of the 2-8 GHz band. Once enabled by the operator, the invention automatically scans the programmed frequencies, collects and processes data for each frequency, and displays results on the invention""s display. The display connects the individual measurement points with a line for clarity and then repeats the measurement process until stopped by the operator. This allows the user to make adjustments to the DUT and immediately observe the results.
The invention sends frequency information to a digitally controlled frequency synthesizer via the IEEE-488 bus. Software drivers for Hewlett Packard Models 8672A and 836x series synthesizers are supplied. The number of frequencies and the frequency steps chosen must be compatible with the particular synthesizer used. The invention also controls the amplitude of the synthesizer output. It is recommended that the synthesizer amplitude be set to at least +13 dBm. This will return the nominal reference input amplitude of +10 dBm after a 3 dB loss in the passive power splitter.
The invention comprises a commercial off-the-shelf (COTS) personal computer (PC) and TSC-designed electronic modules housed in a rugged aluminum, portable chassis. The host processor is a Pentium III with a 15.1-inch, high brightness, active matrix TFT. The integral keyboard/pointing device is sealed and water-resistant. A plug-in IEEE-488, GPIB controller is used to control the external synthesizer.
In addition to the COTS components, the chassis houses two, TSC-designed plug-in circuit boardsxe2x80x94a RF Module and a Digital Signal Processor (DSP). These modules include phase shifter and vector demodulator circuits, two 12-bit, 50 MHZ analog to digital converters, and a high-performance DSP that implements rotating phase reference (RPR) algorithms to generate the outputs.
Once enabled by the operator, the invention automatically steps through the specified frequencies and graphically displays the results on its display screen in a matter of seconds. Numerical results can be saved to disk for further review and analysis or can be viewed on the screen at any time.
The invention""s graphical user interface was developed using LabVIEW for Windows. It provides familiar-looking, virtual instrument panel that includes indicators and controls that are manipulated using the keyboard and/or pointing device (mouse).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and constitute a part of the specification, illustrate certain embodiments of the invention, and together with the detailed description, serve to explain the principles of the present invention.